Metals typically are extracted from raw materials, such as metal oxides, and thereafter separated from other materials either used for or generated by the extraction process. Solvent extraction at atmospheric pressure following dissolution of solids with an acid is a widely used technique for extracting metals and metal oxides from solid materials. However, conventional acid dissolution followed by solvent extraction processes requires large amounts of solvents and acids. Those same solvents and acids often become waste, and waste treatment and disposal presents an important environmental problem, particularly for radioactive solid wastes. Removing radioactive materials and metal contaminants from wastes generated by mines and nuclear plants would facilitate safer and cheaper disposal of the remaining waste products. Current methods for decontaminating such wastes are infeasible on an industrial scale because of the large quantity of secondary acid and solvent waste generated by such methods.
Recently, supercritical fluids comprising a chelating agent have been proposed for chelation and dissolution of metals and metal oxides without the use of either organic solvents or aqueous solutions. Various features of supercritical fluid extraction of metals and metalloids are disclosed in Dr. Chien Wai et al.'s U.S. Pat., Nos. 5,356,538, 5,606,724, 5,730,874, 5,770,085, 5,792,357, 5,965,025, 5,840,193, 6,132,491 and 6,187,911 (“Wai's patents”). Wai's patents are incorporated herein by reference. Wai's patents disclose various features for extracting metalloid and metal ions from materials by exposing the materials to a fluid solvent, particularly supercritical carbon dioxide, containing a chelating agent.
Despite these prior known processes, there are still some disadvantages associated with these and other more traditional purification processes for metals, such as uranium. These disadvantages include: (a) low yields of purified metals and low overall efficiency; (b) time consuming steps; (c) the creation of undesirable waste streams; and (d) slow extraction rates.
A need therefore exists for an environmentally safe method for separating and/or purifying metals from other metals, metalloids and/or impurities. A further need exists for a method which is both efficient and provides for a greater yield of the extracted and purified metals.